1. Field of the Invention
The present invention relates to a laminated optical film. An optical film of the present invention may be used independently or may be used in combination with other optical films as various optical films, such as retardation films, viewing angle compensation films, optical compensation films, elliptically polarizing plates (including circularly polarizing plates), and brightness enhancement films. A laminated optical film of the present invention is especially useful when it is laminated with polarizing plates to be used as elliptically polarizing plates.
In addition, the present invention relates to an image viewing display such as a liquid crystal display, an organic EL (electroluminescence) viewing display, a PDP using the laminated optical film, the elliptically polarizing plate, and the like. A laminated optical film and an elliptically polarizing plate of the present invention may be applied for various liquid crystal displays etc. as described above, and may be especially suitably used for reflective and transflective type liquid crystal display that can be mounted in portable information and telecommunications instruments, personal computers, etc. It is also suitable for mounting to liquid crystal displays in TN (Twisted nematic) mode, OCB (Optically compensated bend), and homogeneous mode as liquid crystal displays.
2. Description of the Related Art
Conventionally, many optical films comprising various kinds of polymer materials have been used for the purpose of improving picture-quality in image viewing displays, such as portable information and telecommunications instruments, liquid crystal monitors, liquid crystal televisions, organic EL viewing displays. For example, performing stretching process for polymer films having birefringence produces such optical films. Among them when a direction wherein refractive indexes of refraction in a film plane gives a maximum is defined as an X-axis, a direction perpendicular to the X-axis is defined as a Y-axis, a thickness direction of a film is defined as a Z-axis, and refractive indexes in each axial direction are defined as nx, ny, nz, respectively, an optical film wherein a Nz coefficient expressed in a formula of (nx−nz)/(nx−ny) is controlled may preferably be used in order to widen a viewing angle of image viewing displays, such as the above-mentioned liquid crystal displays.
An Nz coefficient preferable for optical films depends upon modes (TN, VA, OCB, IPS modes, etc.) of the liquid crystal displays. Therefore, in order to obtain optical films having a required Nz coefficient polymer materials having superior film workability and birefringence that may easily be controlled into a desired Nz coefficient are suitably selected for use. For example, since optical films satisfying a relationship of Nz coefficient ≦0.9 may control indexes of refraction to be at least nz>ny, polymer materials having such indexes of refraction and developing birefringence are suitably used.
Since optical films satisfying Nz coefficient ≦0.9 may advantageously developing superior birefringence, for example, they may be obtained by stretching polycarbonate resin films including a unit of 2,2-bis (4-hydroxyphenyl) propane as polymer films (See Japanese Patent Laid-Open No. 5-157911 official report). The polycarbonate resins are preferable from a viewpoint of having high transparency and moderate heat resistance. However, optical films obtained by stretching of polycarbonate resin films have a large birefringence change when stress is applied; that is, they have a large photoelastic coefficient. Therefore, there occurs a problem that the optical films easily cause a large degree of unevenness when they are adhered to polarizing plates. Moreover, in recent years, upsizing of liquid crystal panels, such as in liquid crystal televisions, increases stress that works on panels, and therefore optical film materials having smaller change of retardation (change of birefringence) is increasingly required. Moreover, the optical films have such problems that exhibit large retardation variation, under use environment after adhered onto viewing displays. Since they had this problem, the optical films were not suitable for application in recent years wherein high heat resistance, and high temperature and high moisture resistance were required.
On the other hand, as polymer materials having a comparatively small photoelastic coefficient, for example, norbornene resins are known (See Japanese Patent Laid-Open No. 2000-56131 official report). However, although the norbornene resins have a small photoelastic coefficient, they simultaneously show a characteristic to have a small birefringence, providing a limitation to retardation given by stretching process. Especially, control of three dimensional refractive index satisfying a relationship of Nz coefficient ≦0.9 is difficult.
Conventionally, broadband retardation plates are suitably used that have functions as a quarter wavelength plate or a half wavelength plate with respect to incident light with wavelength area of broadband (visible light range) for reflective and transflective type liquid crystal displays etc. As this broadband retardation plates, laminated films obtained by laminating two or more polymer films having optical anisotropy in a state of optical axes being intersected with each other are proposed. In these laminated films, broadband characteristics are realized by making optical axes of two-layered or two or more sheets of stretched films intersect with each other (for example, refer to Japanese Patent Laid-Open Publication No. 5-100114, Japanese Patent Laid-Open Publication No. 10-68816, Japanese Patent Laid-Open Publication No. 10-90521).
However, even when the broadband retardation plates described in the above-mentioned Patent Literatures are used, there is a defect of gradation inversion wherein observation of displayed picture in diagonal (upward, downward, right-hand and left-hand) directions with respect to normal line of a screen varies hue of the displayed picture, or gives inversion between white images and black images.